1. Field of the Invention
The present invention relates to a comprehensive system for the production and harvesting of algae including as one component a photobioreactor for optimizing algae growth for use in carbon dioxide sequestration and biomass and biofuel production.
2. Brief Description of the Prior Art
Algae perform biosynthesis of carbon dioxide. In the past, algae have been commercially cultivated in ponds, raceways, tubes, liquid membranes and panels.
In pond cultivation, concrete-made open cultivation ponds are built outdoors filled with water and algae are cultured using available sunlight. A high concentration of algae cells form a thick blanket of algae on top, which inhibits sunlight penetration. Unless the algae cell density is reduced in the top few centimeters, the total algae photosynthetic efficiency is lowered. Hence it is necessary to stir the water in the pond and to keep the depth of pond to about 15 cm or less. Stirring requires a large amount of energy and the required land for the shallow ponds is vast. Further, since the cell density in the ponds must be kept low, the biomass must be harvested from a huge amount of low density culture solution. In addition, because the cultivation pond is open and located outdoors, dust, waste and airborne microorganisms may contaminate the pond making it impossible to maintain an algae culture of high purity and quality. The cultivation pond is also subject to temperature fluctuations and, in certain locations, freezing.
Open ponds can be replaced with raceway type vessels which consist of a long, narrow open cultivation tank with a divider down the middle to form a circular path. The algae are then mechanically circulated around the raceway in a continuous circle. This system creates an improved circulation system, but the photosynthetic rate of algae is low, similar to that in a cultivation pond. Light is not efficiently used, carbon dioxide conversion is low and the mechanical stirring of the algae results in broken cells due to shear stress. This causes a reduction of cellular activity resulting in a low propagation rate. The negative aspects of limited sunlight, temperature fluctuations and culture contamination still apply in the open raceway system.
In tubular type cultivation, algae are cultivated in a light transmission tube. Algae cultivation using this apparatus protects the culture from contamination by foreign microorganisms, leading to a high culture concentration. In due course of operation, however, algae attach to the internal wall of the tube reducing light being passed through the tube which reduces algae cultivation. Removal of the algae attached to the internal wall of the tube is difficult. Growth of the algae is further inhibited by the presence of oxygen which is a byproduct of photosynthesis and collects in the tube.
In a liquid membrane-forming cultivation apparatus, a dome-shaped, light-passing lid body is installed on a top face of a small cultivation vessel. A culture solution is jetted from a bottom to an internal face of an apical part of the dome-shaped lid body to form a liquid membrane on the internal face of the lid body where it is irradiated with light. This system requires a circulation pump for continual formation of the liquid membrane and is not suitable for mass cultivation.
A panel type cultivation apparatus makes use of a thin box-like apparatus, which is prepared by using two resin-made panel boards, with inclination. The panel type also relies on sunlight for algae photosynthesis. The panel is a closed system similar to the tubular type cultivation apparatus and, accordingly, has an advantage of no contamination of the culture solution by stray microorganisms, dirt and waste. However, the buildup of oxygen once again stays in the apparatus and inhibits algae growth. When solar energy is used for photosynthesis another problem arises. As the altitude of the sun changes throughout the day and through the seasons, the sunlight incident angle to a surface of the apparatus changes resulting in an insufficient total amount of sunlight received per unit area.
Algae accumulate useful substances in their bodies by photosynthesis. Photosynthesis may be promoted by increasing the light-receiving area of the cultivation apparatus, efficiently stirring the culture solution, adjusting the thickness or a depth of the culture solution, removing and cleaning the algae cells attached to the internal surface of the cultivation apparatus, regulating the temperature, preventing contamination with dust, waste, bacteria, and other algae species and so forth. None of these prior art systems optimize algae growth. For example, with ponds the light-receiving area can be increased only by increasing the area of the pond. Stirring breaks up the cells. Temperature can be regulated by pumping cold water into the prior art apparatus. This however results in a diluted solution being harvested with low algae density.
Hence the conventional methods described above each have pros and cons and none provide solutions to all the issues involved in large scale emissions (e.g., +25,000 cfm) of greenhouse gases.